Orthopedic and neurological surgeries involve drilling bone to accurate depths. For example, it is often desired that the drill bit does not contact and/or damage the soft tissues and nerves beyond the far cortex of the bone. See, for example U.S. Pat. No. 5,601,557, the disclosure of which is hereby incorporated herein by reference. The '557 patent discloses drilling a hole through the hard outer shell of bone into the less dense cancellous bone followed by a surgeon inserting a small, but resilient device (an “anchor”) by pressing it into the smaller drilled hole. The anchor compresses as it passes through that hole, but once in the softer cancellous bone, the resilience causes it to expand again, causing it to stay in place. The attached anchor is then used to secure tissues to the bone, e.g., for repair of damaged ligaments and the like.
Orthopedic surgery and trauma care often require bone drilling when repair of fractures is indicated. The holes resulting from such drilling can accept screws to hold implants such as plates, prostheses or to exert pressure between broken and subsequently reduced bone fragments. The screws inserted have to fit properly, such that no unused space remains, which could otherwise cause problems for the patient. Conventional techniques used for measuring the length of such holes is often done by using a mechanical slide gauge, which is time consuming, and often requires the use of repeated x-ray-control and corrections. See for example, U.S. Pat. No. 8,092,457, and U.S. Patent Publication No. 2008-0255572, the disclosures of which are hereby incorporated herein by reference.
Most orthopedic and/or neurological hand-held drill devices do not provide any means for automatic depth control. Most procedures rely entirely on the surgeons' judgment and skill to estimate the bone cutter depth. This invention provides an automatic depth control for bone drilling.
The precision bone drill of the present invention is an improved and modified version of a device known as the PathFormer device. The PathFormer device is described in U.S. Pat. No. 7,848,799, the disclosure of which is hereby incorporated by reference. See also FDA 510(k) Application No. K052770, approved for the PathFormer device.
The PathFormer device (see FIGS. 1 and 2) is designed and intended for cutting through finger and toe nails, e.g., to release fluid accumulated in the underlying nail bed, for relieving pressure from subungual hematomas (including black toe). The PathFormer device is an electrosurgical hand-held device that cuts holes in fingernails and toenails using mesoscissioning technology. It cuts the nail with a microcutting tool, using tissue impedance as the feedback mechanism for stopping the cutting intervention.
Using the PathFormer device for cutting through finger and toe nails (trephination), one or more microconduits are drilled to specific depths without penetrating the nail bed. The nail plate is cut with a 400 μm diameter tissue cutter in the hand-held device which contains two small electric motors. One motor rotates the cutter, and the second moves the cutter up and down. The motors are powered by a portable 9V power supply. The cutting motor is connected to an electronic control that measures the electrical impedance between the rotating cutter and an electrocardiogram electrode (i.e., counterelectrode) on the patient's skin. This control can be calibrated to reverse the cutting motor at the detection of a pre-set electrical impedance.
Because the nail plate is highly keratinized, it normally has a high electrical impedance, approximately 5 Mohms, as calculated in preliminary experiments. In contrast the normal impedance of the nail bed is much lower, in the 10-20 Kohms range, largely because of the higher saline content of nail bed tissue. During the drilling process, the removal of each successive layer of the nail plate results in a reduction in electrical impedance at the site of the hole. When the measured electrical impedance has decreased to the trigger impedance, the cutting tool instantaneously and automatically pulls away from the nail.